The invention generally relates to the detection of particles, and more specifically to the measurement of dust particle concentrations and size distributions.
When inhaled, aerosol particles can deposit on the respiratory track and cause adverse health effects. Hence, industry and government have recognized the importance of measuring and monitoring aerosol concentrations in the environment or workplace so that proper measures can be taken to reduce potential health risks. Pertinent monitoring applications include but are not limited to commercial building or home air quality monitoring, industrial/occupational hygiene surveys, outdoor ambient/site perimeter monitoring for dust control operations, and engine emission studies. Some industrial processes require knowledge of the particulates in the environment, including environments having a sparse population of particles (e.g., semiconductor clean room manufacturing or pharmaceutical drug processing) as well as environments having an extensive presence of particle populations (e.g., dry powder manufacturing processes). In 1987, the United States Environmental Protection Agency (EPA) revised the National Ambient Air Quality Standards (NAAQS) and started to use mass of particles with aerodynamic diameters less than approximately 10 μm (hereinafter “the PM10”) as the particulate matter (PM) pollution index. The PM10 is an index of the PM that can enter the thorax and cause or exacerbate lower respiratory tract diseases, such as chronic bronchitis, asthma, pneumonia, lung cancer, and emphysema. It was later determined that PM concentrations in the air, as indexed by the mass of particles with aerodynamic diameters less than approximately 2.5 μm (“PM2.5”) was more closely associated with the annual mortality rates than with the coarser PM10. In 1997, in its next revision of the NAAQS, the EPA promulgated regulations on PM2.5.
The American Conference of Governmental Industrial Hygienists (ACGIH) has also established sampling conventions of respiratory, thoracic and inhalable aerosols, defined as particles having aerodynamic diameters of less than 4 μm, 10 μm, and 100 μm respectively. Inhalable particles are those capable of entering through the human nose and/or mouth during breathing. Thoracic particles are the inhaled particles that may penetrate to the lung below the larynx. Respiratory particles are the inhaled particles that may penetrate to the alveolar region of the lung. A discussion of the various sampling conventions are found at National Primary and Secondary Ambient Air Quality Standards, 40 Code of US Federal Regulation, Chapter 1, Part 50 (1997) and Vincent, J. H., Particle Size-Selective Sampling for Particulate Air Contaminants Cincinnati, ACGIH (1999), both of which are hereby incorporated by reference except for explicit definitions contained therein.
While the aforementioned standards and conventions are based on the aerodynamic diameters of particles, it is understood that size segregated mass concentration groupings (e.g., PM1, PM2.5, PM10, respirable, thoracic and inhalable) may be based on the optical particle diameters instead of the aerodynamic diameters for purposes of the instant application. That is, PM2.5 (for example) may approximate particles having an aerodynamic diameter of less than approximately 2.5 μm or particles having an optical diameter of less than approximately 2.5 μm. Particle mass measurements can be achieved in real time using a photometer if the aerosol is primarily a fine aerosol (approximately between 0.1- and 4-μm). The photometer is a device that produces an electrical signal that varies with the intensity of scattered light received from a particle or an ensemble of particles in the interrogation volume region. The photometric signal can be approximately correlated to particle mass. The photometer may also be sensitive to a wide dynamic range of particle concentration. For example, the TSI Model 8520 DUSTTRAK photometer measures a particle mass concentration range of 0.001- to 100-mg/m3 over the particle size range of 0.1- to 10-μm.
Government regulations exist in many countries that require monitoring of various pollution parameters. Instrumentation for measuring these parameters according to regulations tends to be expensive, on the order of $15K-$100K, and the operational costs can be very expensive as well. Due to the cost, there are few instruments in a given geographical area to indicate air quality. Since pollution sources can be localized, there is a great deal of interest in measuring at many points within a geographical area. Sensors for many distributed measurements must be inexpensive but still give a fairly accurate measurement compared to higher precision instruments. There are low-cost techniques for giving an indicative measurement of particulate mass but they are very limited in accuracy. The most common are photometric sensors that measure an ensemble of light scattered from a light source (usually a laser, LED or other source of intense light) and detected by a photodetector (usually a photodiode or other sensitive light detector). The most common low-cost option is an LED light source and photodiode detector. These types of sensors incorporate electronics that convert the signal from scattered light to an electrical signal that can be processed to give a user an indication of particle mass on a display.
In other instances, gravimetric sampling is used which consists of collecting particles, usually over a long period of time such as 24 hours, on a pre-weighed filter. The weight of the filter and particles are then measured. The difference in weight between the filter before and after sampling provides the weight of the particles in a given period of time for a given sampling volume. Some of the advantages of filter sampling include: 1) it is relatively inexpensive to set up and implement and then interpret the data; and 2) the concepts of measurement based on first principles are easier to grasp and share with other interested parties. Other the other hand, gravimetric filtering does have a higher total cost of measurement which includes a great deal of labor cost making it very expensive, trying to measure low particle concentrations can be very difficult since the measurement is a difference in two weight conditions (unused and used filter), poor time resolution, the process is labor intensive and operator error is a possibility, and particles may evaporate before they are weighed.
Photometer sensors tend to require frequent calibration of zero and span due to sensor drift with temperature, humidity or other outside factors. Some disadvantages of various photometers are: (1) only the total mass is measured (no particle size segregated mass information is provided); (2) the photometric signal is dependent on particle properties such as size, shape and refractive index, thus requiring different calibration factors for different aerosols; (3) photometers are typically more sensitive to particles having diameters close to the wavelength of the light source, with a precipitous drop off in signal per unit mass for particles outside of this size range; and (4) photometers can underestimate particulate mass if the sampled aerosol contains particles larger than 4 μm.
One instrument that measures particle size dependent number concentrations in real time is the optical particle counter (OPC). In an OPC, individual particles pass through an interrogation volume that is illuminated by a light beam. The light scattered by each particle is collected on to a detector to generate an electrical pulse. From the pulse height and/or pulse area (i.e. the intensity of the scattered radiation) one can infer the particle size based on prior calibration. Because the size inferred from the OPC depends on the particle optical properties, the inferred parameter is often referred to as the “optical equivalent particle size.” Some advantages of the OPC are: (1) particles may be counted with high accuracy for low particle concentrations; (2) favorable signal to noise ratios for particle sizes greater than 1 μm; and (3) low cost. However, the inferred particle optical size may not be the same as the actual or geometric particle size because the determination depends on the particle shape and refractive index assumptions. Additional errors may arise when converting the particle size distribution to a mass concentration if the particle density is incorrectly assumed. Furthermore, OPCs typically underestimate particle concentration when multiple particles are present in the interrogation volume region (a condition often referred to as “coincidence error”). Accordingly, OPCs are typically only used in relatively clean environments. An example is the TSI Model 9306 OPC, which counts 95% of particles at a number concentration of approximately 200 particles/cm3 or mass concentrations less than 1-mg/m3. The counting efficiency of the Model 9306 drops quickly as concentration increases above these limits.
In summary, filter sampling provides first principle mass measurement, but has poor time resolution and it does not provide particle size information. Obtaining size segregated mass concentration measurements may require the procurement and maintenance of multiple instruments. The photometer measures a wide particle concentration range, but it does not provide particle size information and may be relatively insensitive to particles having diameters greater than approximately 4-μm. An instrument and system that can provide size segregated particle mass concentrations information in real time and over a wide range of mass concentrations and a wide geographical area at a competitive cost and simplifies remote calibration of would be a welcome improvement.